<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<style>
  * {
    padding: 0;
    margin: 0;
  }

  html,
  body {
    height: 100%;
  }

  .box {
    width: 100%;
    height: 100%;
    display: flex;

  }

  .item {
    height: 100%;
  }

  .item-child {
    width: 100px;
    height: 100%;
    background-color: orange;
  }

  .item2 {
    flex: 1;
    height: 100%;
    background-color: red;
  }
</style>

<body>

  <div class="box">
    <div class="item">
      <div class="item-child">12</div>
    </div>
    <div class="item2">item2</div>
  </div>
</body>

</html>